In physics class, the girl to boy ratio is $2$ to $9$. If there are $45$ boys, how many girls are there?
Explanation: A ratio of $2$ girls to $9$ boys means that there is a group of $2$ girls for every group of $9$ boys. If there are $45$ boys, then there are $5$ groups of $9$ boys each. Because we know the ratio is $2:9$ , there must be $5$ groups of $2$ girls each. There is a total of $10$ girls in physics class.